1. Field of the Invention
This invention relates to methods, compositions, and devices for determining the specific gravity of a liquid. In particular, this invention relates to means for determining the specific gravity of an aqueous liquid. The present invention finds application in the determination of the specific gravity of body fluids such as urine.
Specific gravity is defined as the ratio of the density of a substance to that of a standard substance. Where the substance whose specific gravity is to be determined is in a liquid form, the standard substance generally is water. In the context of this disclosure, a liquid whose specific gravity is to be determined shall be defined as a mixture of substances whose aggregate is in a homogenous liquid state. The substance in the liquid that is present in the greatest amount shall be defined as the solvent or solvent component with the remaining substance or substances being defined as the solute.
Specific gravity determinations are used in the fields of science, medicine, and industry for a wide variety of purposes. For example, it is often quite useful to ascertain the specific gravity of an unknown liquid to aid in identification thereof. The determination of the specific gravity of body fluids, particularly urine, is a part of nearly all routine urinalyses. Urine consists of water as solvent and various dissolved solids as solute. Only a minor portion, if any, of the solute component of urine consists of liquid substances. Thus, the specific gravity of urine indicates the relative proportions of dissolved solid components to the total volume of the specimen tested and therefore reflects the relative degree of concentration or dilution of the specimen. Under appropriate and standardized conditions of fluid restriction or increased uptake, the specific gravity of a urine specimen measures the concentrating and diluting abilities of the kidney.
Normal urinary specific gravity ranges from 1.003 to 1.035, but usually remains between 1.010 and 1.025. Specific gravities below 1.010 can be indicative of diabetes insipidus, a disease caused by the absence of, or impairment to, the normal functioning of the antidiuretic hormone. Low specific gravity may also occur in patients with glomerulonephritis, pyelonephritis, and various renal anomalies. Specific gravity is high in patients with diabetes mellitus, adrenal insufficiency, hepatic disease, and congestive cardiac failure. Therefore, urinary specific gravity determinations are useful in routine urinalysis as a screening procedure for detecting potentially abnormal clinical conditions.
2. Description of the Prior Art
By its definition, the most straight forward method for determining the specific gravity of a liquid is to determine the ratio of the weight of a given volume of the liquid to the weight of the same volume of water under standard conditions. Such a method, however, requires precise volumetric and gravimetric techniques. More often, the specific gravity of liquids is determined using pycnometers or gravitometers. Such instruments have as their principle the fact that, if two manometers containing liquids of different densities are connected to a common source of suction, the heights of the liquids are inversely proportional to their densities, and hence their specific gravities.
In the clinical laboratory, urinary specific gravity is determined in many ways. The specific gravity of urine is usually determined with a urinometer. A urinometer is a weighted bulb-shaped device having a cylindrical stem which contains a scale calibrated in units of specific gravity. The device is floated in a cylinder containing the urine. The depth to which the device sinks in the urine indicates the specific gravity of the urine and is read on the scale at the surface of the urine. The urinometer is sensitive to temperature requiring an adjustment of 0.001 units for each 3.degree. C. difference between the calibrated temperature of the device and the temperature of the urine specimen.
All of the methods and devices mentioned hereinabove for determining the specific gravity of a liquid require a relatively large sample volume in order to conduct the test. The need for a several milliliter volume of sample sometimes requires that an additional urine specimen must be obtained from the patient, thereby destroying the continuity of routine urinalysis. It is most desirable to obtain all of the routine urinalysis results from a single sample in order that the relationships between the test results may be properly analyzed.
Through the use of a refractometer, urinary specific gravity determinations may be carried out using as little as one drop of urine. Since in urine the solute consists essentially of only dissolved solids, the refractive index of urine closely correlates with its specific gravity. Small hand refractometers are available specifically designed for determining urinary specific gravity.
Many other diverse techniques are available for determining the specific gravity of a liquid. All of the known techniques require a bulky instrument or device of one sort or another which has to be consistently cleaned, maintained, and adjusted in order to produce reliable results. All such techniques also are based strictly on physical measurements such as measurements of volume, weight, height, and refractive index.
Routine urinalysis, as practiced at the present time, involves three basic areas of investigation: a determination of the presence or absence of substances such as glucose, protein, occult blood, ketones, and so forth; a determination of specific gravity; and a microscopic examination of the urinary sediment. The first area of investigation usually involves the testing of the urine specimen with indicator papers or strips comprising reagent pads responsive to the urinary constituents to be determined. Indicator strips, usually in the form of single strips carrying multiple reagent pads responsive to the different urinary constituents to be determined, are dipped momentarily into the urine specimens, and the resulting color responses are compared to a color chart. Under present technology, separate analytical steps must be undertaken to determine urinary specific gravity and to microscopically examine the urinary sediment.